The present invention relates to the diagnostic use of a new marker molecule for renal pathology that appears in both blood and urine, to detect the presence of significant renal injury, hereinafter used in the sense of injury to constituent cells of the kidney, including but not limited to renal tubular cell injury, in a human being or mammal. The marker molecule is neutrophil gelatinase-associated lipocalin (NGAL), also known as neutrophil lipocalin (NL; HNL in the case of human neutrophil lipocalin), lipocalin 2 (LCN2), 25-kDa alpha2-microglobulin-related protein (in the rat) or 24P3 (in the mouse). In the rat, it has also been referred to as neu-related lipocalin (NRL), as its gene is overexpressed in mammary tumors initiated by the neu (HER2/c-erbB-2) oncogene (Stoesz and Gould, 1995). NGAL is a 25-kDa glycoprotein first isolated from the granules of neutrophil polymorphonuclear leukocytes (Triebel et al., 1992; Kjeldsen et al., 1993). It contains a disulfide bridge and forms a proportion of dimers and a smaller proportion of trimers. Part of the total secreted NGAL is associated with 92-kDa human neutrophil type IV collagenase, also called matrix metalloproteinase 9 (MMP-9) or gelatinase B, either as an NGAL monomer forming a complex of apparent kDa 115, (Monier et al., 2000; Yan et al., 2001) or as an NGAL dimer, forming a complex of apparent kDa 125 (Yan et al., 2001). These complexes have been identified in the urine of patients with a variety of cancers, including cancers of the prostate, bladder, kidney and breast.
NGAL was initially disclosed as a marker of neutrophil activation, being released into the blood when invading microorganisms, in particular pyogenic bacteria, cause degranulation of the neutrophils and exocytosis of the granule proteins. As such, the measurement of elevated levels of NGAL in a serum sample from a human is believed to indicate that the individual is suffering from an inflammatory process, especially one caused by a bacterial infection (U.S. Pat. No. 6,136,526; PCT application WO95/29404). In this respect, but in contradiction to its claimed specific derivation from neutrophils, NGAL (24P3) was identified as an acute phase protein of type 1 in the mouse, where expression was mainly located in the liver during the acute phase response (Liu and Nilsen-Hamilton, 1995).
U.S. Patent Application 2004/0219603 discloses the use of NGAL as a urinary biomarker for detecting the early onset of renal tubular cell injury. U.S. Patent Application 2005/0272101 discloses the use of NGAL in blood serum for the same purpose. However, neither disclosure describes how renal tubular cell injury can be discriminated from systemic inflammation, or bacterial infection, or cancers as the cause of the elevated NGAL level. The present inventors have previously filed a PCT application WO2006/066587 which discloses how NGAL levels in urine or blood plasma or serum can be separated into lower elevations that are not diagnostic of renal injury and higher elevations that are, by defining cutoff levels of NGAL in these fluids that must be exceeded in order for the NGAL measurement to be diagnostic of renal injury leading to ARF or ARD. WO2006/066587, which is hereby incorporated into the present application by reference in its entirety, takes into account the fact that NGAL levels in bodily fluids may also be elevated in inflammations, infections and certain cancers, but usually to lower values than those associated with renal injury leading to ARF or ARD. However, a small number of patients with severe infection and/or cancers may show NGAL levels that are above the cutoff level used to diagnose renal injury, even though they do not develop renal injury during their hospitalization, while a few patients develop a minor degree of ARD even though their NGAL levels did not exceed the cutoff. When a cutoff level is used, these two types of borderline cases may give rise to false positive diagnoses of renal injury or false negative diagnoses of absence of renal injury, respectively.
It is the purpose of the present invention to provide a reliable means of distinguishing between rises of NGAL in bodily fluids that are due to renal injury and those that are due to non-renal causes, without resorting to the use of a cutoff value for the concentration of NGAL in a given bodily fluid.